Aldehyde and polyisocyanate pre-polymer resins are generally known materials useful for coatings, binders, and other materials. Aldehyde base pre-polymer resins are usually made from formaldehyde, though other aldehydes may be used, in combination with other synthetic materials like urea, melamine, phenol, resorcinol, furfural, and natural materials like lignin, tannin, protein or amino acids, or various combinations of any of them. Isocyanate based pre-polymer resins may comprise aromatic isocyanates like toluene diisocyanate (TDI) or diphenyl methane diisocyanate (MDI), while aliphatic isocyanates may comprise hexamethylene diisocyanate (HDI), isophorone diisocyanate (IPDI), or hydrogenated MDI (H12-MDI), or combinations of any of them. Aldehyde and isocyanate pre-polymer resins find broad usage in various applications such as wood composites, paper impregnation, fibrous composites, ceramic composites, foundry binders, insulation foams, and coatings among others.
Pre-polymer resins and the polymers made from them are expected to meet many performance requirements such as ease of application as a spray, by coating, in impregnation, or in mixing. For example, they should be stable prior to use, able to be used safely and sufficiently stable with good “pot life” during application so as to not cause processing difficulty. They may need to be able to release from a variety of materials and be easy to clean up. The conversion from pre-polymer resin to cured polymer should be able to be done under reasonable conditions of time, temperature, and pressure so as to maintain economically efficient industrial production. They may need to be able to cure in the presence of moisture as bound water or steam vapor. And the cost of the pre-polymer resins and processing them must be economically viable for commercial usage.
Among the aldehyde and isocyanate pre-polymer resins the phenol-formaldehyde (PF) and MDI pre-polymer resins and the respective polymers made from them satisfy many of these performance requirements. In spite of the many advantageous features of PF and MDI pre-polymer resins, their disadvantages have proven difficult to overcome, especially those of the relatively slow PF reactivity and the relatively expensive cost of MDI. Researchers have attempted to overcome these drawbacks with modifications to the basic PF or MDI pre-polymer resins, while others have combined them to draw upon the best attributes of each polymer.
Resorcinol-formaldehyde (RF) and Phenol-resorcinol-formaldehyde (PRF) resins are used to bond wood at ambient temperatures, while PF resin may be resorcinol modified, taking advantage of the high reactivity of the resorcinol. The cost of resorcinol, however, is much greater than phenol, and limits their usages to a few specialized applications.
Esters, organic carbonates, and lactones may act as accelerators for PF resins. U.S. Pat. No. 7,049,387 B2 discloses a method of accelerating the cure of a phenol-aldehyde resin with conjoint use of a cyclic carbonate and an amine. The cure rate can be further accelerated by the addition of a resorcinol source. Alkylene carbonates, polyalkylene amines and polyalkylene glycol amines are preferred, as is a formaldehyde deficient RF resin. In the example given, various accelerated resin mixes of a PF resin with an F:P molar ratio of 1.50 still requires in between 36 and 60 seconds in a stroke cure test run at 150° C. to cure.
Various efforts have combined PF resin and MDI in wood adhesives, some by separate component application, some by direct mixing and others by chemically blocking one of the components. U.S. Pat. No. 6,214,265 discloses a process for binding wood using an adhesive composition comprising a polymethylene polyphenylisocyanate and a solid PF resole. Composites are made by pressing at 350° F. for 4.5 minutes. In a related patent, U.S. Pat. No. 6,294,117 discloses a similar process except using a solid PF novolac instead of a solid resole. Wood composites are made by pressing at 350° F. for 4.5 minutes. Both patent disclosures are directed to formation of the composite within 2 to 10 minutes at temperatures of 120 to 225° C. U.S. Pat. No. 6,224,800 discloses the use of solid urea or melamine to extend polymethylene polyphenylisocyanate in wood composite binders. Particleboard panels are bonded at 350 or 400° F. for 4.5 minutes.
Others have attempted to implement ambient cure systems for wood composites. One such project is described in, “Rapid, Low-Temperature Electron, X-ray, and Gamma Beam-Curable Resins”, which advocates development of rapid, low-temperature electron beam-curable resin systems for wood adhesives, estimating that such systems would offer a potential energy savings to the wood composites industry of 65 Trillion BTU's/year at full market penetration. The reduction of curing temperatures from 450° F. to 250° F. possible with beam-curing systems also offers the potential of reducing unit capital costs and doubling throughput. The lower curing temperatures would also decrease process emissions by reducing volatile organic compounds (VOCs). Another project, “Development of a novel adhesive for cold-press production of Laminated Veneer Lumber” attempts to develop novel moisture-curing polyurethane adhesives for the cold-press manufacture of laminated veneer lumber made from high moisture content wood with the project's focus being on combinations 100% organic, isocyanate-reactive polyurethanes combined with reactive latex crosslinked with polyisocyanate.